A couple of weeks ago, in the wake of the release of the final rule for the U.S. Environmental Protection Agency's Phase 2 Greenhouse Gas regulations, I told Fleet Owner readers the different ways I thought that ruling would affect powertrain development over the next decade.

As I noted, one of the likely new technologies we'll start to see in the next decade will be hybrid-electric drivetrains, which will be optimized to give heavy duty trucks a big torque boost in lower gears to get the vehicle moving in a much more efficient manner.

Hybrid-electric drivetrains aren't new, of course. They first appeared in the late 1990s, most notably as a joint medium-duty offering from Eaton and International Trucks. In this incarnation, hybrid-electric drives functioned much as they likely will in the future, providing instantaneous electric torque to drive wheels. 

But, designers at the time also envisioned hybrid trucks as rolling power stations. The vehicles were outfitted with massive, heavy, battery packs that stored powered and enabled trucks to run a wide array of equipment with it. In most cases, this showed up on bucket trucks, or mobile drill rigs or service trucks. The idea seemed so promising at the time that light-duty OEMs quickly followed suit, with GM, in particular, offering hybrid-electric options for Silverado and Sierra pickup trucks. 

Alas, it all came to naught. As it happened, hybrid-electric drivetrains on the medium-duty side of the market proved to be too costly for municipal fleets to purchase in large numbers. Hybrid production never scaled up, and remained prohibitively expensive throughout its production run.

Another problem was maintenance. By and large, the systems were reliable. But when problems did occur, they proved difficult and expensive to repair. The trucks' complex high-voltage electrical systems were particularly tough to trouble-shoot and repair.

In the end, most fleets simply decided the fuel economy gains and modest convenience of having a mobile power pack didn't justify the added costs of the vehicles. Over time, they pretty much faded away.

But now, thanks to the EPA, hybrid-electric drives appear poised to make a come-back. And while I fully understand if you're not turning cartwheels at this bit of news, stop and consider that the hybrid-electric drives of the near future will not be the hybrid-electric drives of 20 years ago.

As is often the case, I felt that after-market suppliers actually ID'd the optimal hybrid-electric drive powertrain configuration.

At the height of trucking's hybrid drive flirtation, after-market suppliers began developing and selling hybrid modification kits that could be installed on a light-duty van or truck transmission to provide a "light" hybrid power boost when the vehicle was getting under way.

These systems did away with the heavy and complex battery packs featured on heavier trucks, and didn't bother with trying to store energy to use later on. The system was simplified and optimized to give the drivetrain a boost at low engine RPMs, and that was it.

I have a feeling this is the route Class 8 OEMs will take when they opt to add hybrid-electric drive lines to trucks as part of their GHG 2 strategy over the next decade.

Clearly, there simply isn't room for large battery systems on Class 8 trucks to be viable. And I don't need to point out that the weight penalty associated with those battery packs is a non-starter for fleets. 

But, a highly-optimized, light-hybrid system that can capture kinetic energy during braking and put that power to use to get heavy trucks up and moving quicker makes an awful lot of sense to me. Remember, fuel burn on a Class 8 truck increases exponentially when it is starting out in lower gears, and then drops off dramatically once the rig is up and moving in higher gears. 

Clearly, the systems will need to be carefully designed to keep weight and space requirements low. And it will need to be a robust system that is fairly easy to maintain and repair. 

But if those goals can be met, my suspicion is that hybrid-electric drivetrains may eventually prove to be a fuel-saving technology that fleets will feel is a winner.